Electromagnetic interference (EMI) filters are often used in switching converters to reduce the amount of EMI. The EMI filter often includes capacitors, such as X capacitors, coupled between the input terminals of the switching converter. When an electrical source is uncoupled from the input terminals of the switching converter, the voltage across the X capacitor may be maintained high and be risky to persons who touch the switching converter after the electrical source has been uncoupled.
To minimize the risk, one or more discharge resistors are normally coupled across the X capacitor to reduce the voltage of the X capacitor to a safe threshold value (e.g. 50V) when the electrical source is uncoupled from the switching converter. However, power is continually dissipated on the discharge resistors even when the electrical source is coupled to the switching converter, which definitely reduces the efficiency of the switching converter, especially under light load and no load conditions.